1. Field of the Invention
This invention relates to pneumatic radial tires, and more particularly to a pneumatic radial tire for use in truck and bus.
2. Related Art Statement
The conventional radial tire for truck and bus comprises a belt of at least three rubberized cord layers each containing steel cords arranged at a relatively small inclination angle with respect to the equator of the tire, cords of which layers crossing with each other, and a carcass composed of at least one rubberized cord ply containing cords arranged substantially perpendicular to the equator of the tire.
In the tire using cords as a reinforcing element for the belt, however, there is a problem that separation failure at the end of the belt (BES) results that in turn produces problems in the retreading and the safety.
The inventors have made studies with respect to the mechanism causing the above problem and confirmed that a crack is created in rubber surrounding a cord end at the end portion of the belt layer due to repetitive deformation of the belt layer during the running, particularly deformation in the face of the belt layer at the generation of side force and grows to connect the other crack created in the adjoining cord and hence causes the separation failure.
Furthermore, the inventors have made studies with respect to a relation of BES occurrence to various factors of the belt layer structure and confirmed that the bending rigidity of the steel cord itself reinforcing the belt layer and the distance between the adjoining cords largely affects the magnitude and creating time of BES.
That is, the bending rigidity of the steel cord itself affects the crack growth at the cord end, while the distance between the adjoining cords affects the easiness of connecting the cracks to each other.
In order to prevent the occurrence of such BES, the inventors have previously proposed in Japanese Utility Model laid open No. 61-206695 that the bending rigidity of the steel cord is made large by restricting a diameter of filaments constituting the steel cord to a range of 0.32-0.42 mm.
According to the above publication, the steel cord comprises a center basic structure (hereinafter referred to as core) and a coaxial layer surrounding it (hereinafter referred to as the sheath) so as to oppose the twisting direction of the filament in the core to the twisting direction of the filament in the sheath for improving the rubber penetrability. However, the filament of the core comes into contact with the filament of the sheath as a point, so that there is still room for the improvement of the resistance to fretting fatigue.